1. Field of the Invention
The present invention relates to a pigment dispersion, and an inkjet ink composition useful for ink jet printing using the pigment dispersion. In addition, the present invention relates to a method of manufacturing the inkjet ink composition, and to an image forming method using the inkjet ink composition.
2. Discussion of the Background
As having been disclosed in Japanese Laid-Open Patent Publications Nos. (hereinafter referred to as JOPs) 63-51485, 63-56575 and 01-198671, inkjet inks in which one or more water-soluble dyes are dissolved in water or a mixture solvent including water and a water-soluble solvent and optionally an additive is added to the solution have been developed and used. However, these inkjet inks (hereinafter referred to as dye type inkjet inks) have the following drawbacks:
(1) recorded images have so poor water resistance as to be blurred when the recorded images contact water; and
(2) recorded images have poor light resistance such that when the recorded images are exposed to light, the color tones of the images change or the image density thereof decreases.
In attempting to solve these problems of the dye type inkjet inks, pigment type inkjet inks which use one or more pigments (e.g., carbon black and organic pigments) instead of the dyes have been disclosed in JOPs. 57-10660, 57-10661, 02-255875 and 02-276876 and Japanese Patent Publication No. 01-15542 (i.e., JOP 56-157468).
When pigment type inkjet inks are used, the resultant recorded images, which have dried on a receiving material, have good water resistance and therefore when the images contact water, the images do not blur (i.e., the images have good water resistance). This is because pigments do not dissolve in water. In addition, since pigments have less reactivity with light than dyes, the images recorded by pigment type inks have better light resistance than those recorded by dye type inks.
Such pigment type inkjet inks are typically prepared by the following method:
(1) a mixture including one or more pigments, a dispersing medium and a dispersant is subjected to a dispersing treatment using a dispersing machine such as ball mills and sand mills; and
(2) the thus prepared pigment dispersion is optionally mixed with one or more additives to prepare a pigment type inkjet ink.
When the pigment dispersion for use in an inkjet ink includes pigment particles having a relatively large particle diameter, the nozzles from which the ink is to be discharged are clogged with the large pigment particles included in the resultant inkjet ink, resulting occurrence of a problem in that the ink is not discharged from the nozzles (hereinafter referred to as a clogging problem), and thereby image qualities of the recorded images, such as resolution and color reproducibility deteriorate. Therefore, the pigment dispersion is typically prepared such that the particle diameter of the pigment particles dispersed therein has a particle diameter not greater than 200 nm, and preferably not greater than 150 nm, to prevent the clogging problem and to produce images having good clearness, color reproducibility (in particular, color reproducibility of secondary colors) and transparency.
In order to manufacture pigment type ink jet inks at a relatively low cost, it is needed to quickly prepare a pigment dispersion including such small pigment particles.
In addition, the thus prepared pigment type inkjet inks in which pigments having a fine particle diameter are dispersed have to maintain the initial dispersion state of the pigments to avoid the ink stop problem mentioned above even when used or preserved under various environments such as relatively high or low temperature environments.
However, there is no dispersant, which can quickly disperse a pigment in a dispersion medium such that the pigment achieves and maintains such a small particle diameter for a long period of time. For example, JOP 56-147863 discloses styrene-acrylic copolymers and styrene-maleic acid copolymers as a polymer dispersant. In addition, JOP 61-83267 discloses an ink which uses a condensation product of sodium naphthalenesulfonate with formaldehyde as a polymer dispersant. In addition, inks including surfactants such as polyethylene glycol alkylphenyl ethers (JOP 05-105837, etc.), sulfates of polyethylene glycol alkylphenyl ethers (JOP 10-168367, etc.) and phosphates of polyethylene glycol alkylphenyl ethers (JOP 10-880509, etc.) as a dispersant have been disclosed.
However, even when these dispersants are used for an inkjet ink, the pigments included in the resultant ink have large particle diameter or the initial particle diameter of the pigments cannot be stably maintained when the ink is used or preserved for a long period of time, and therefore the ink has poor ink discharging property.
In addition, there is no additive which can be used together with a dispersant to improve the dispersibility of the pigment used.
Because of these reasons, a need exists for a pigment dispersion which can be prepared in a short time and by which a pigment type inkjet ink capable of producing good images without causing the clogging problem even when used and/or preserved under various environments can be prepared.
Accordingly, an object of the present invention is to provide a pigment dispersion for use in an inkjet ink composition, which can be efficiently prepared in a relatively short time and by which an inkjet ink composition capable of maintaining a good dispersion state can be prepared.
Another object of the present invention is to provide an inkjet ink composition which can produce images having good image qualities such as good sharpness and color reproducibility even when used and preserved under various environments.
Yet another object of the present invention is to provide a method of manufacturing the inkjet ink composition.
A further object of the present invention is to provide an image forming method by which images having good image qualities such as good sharpness and color reproducibility can be stably recorded even when the inkjet recording is performed under various environments.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be prepared by a pigment dispersion including at least a pigment, a dispersant and a dispersion medium including water, wherein the dispersant has the following formula (1): 
wherein R1 represents an alkyl group having from 1 to 20 carbon atoms, a phenyl group, a substituted phenyl group or a naphthyl group; k is 0 or an integer of from 1 to 7; A1 and A2 independently represent an oxyethylene unit, an oxypropylene unit or a combination of an oxyethylene unit and an oxypropylene unit; and h and j are independently 0 or a positive integer, wherein the total number of h and j is from 31 to 100.
It is preferable that the pigment dispersion further includes one or more of compounds having the following formula (2), (3) and/or (4):
R2-(A3)m-OHxe2x80x83xe2x80x83(2)
wherein R2 represents an alkyl group having from 8 to 14 carbon atoms; A3 represents an oxyethylene unit or a combination of an oxyethylene and an oxypropylene unit; and m is an integer of from 3 to 16; 
wherein M1 represents H, Li, Na, K, a group N+R4R5R6R7 in which R4, R5, R6 and R7 independently represent a hydrogen atom, a methyl group, an ethyl group, a 2-hydroxyethyl group or a 3-hydroxypropyl group; and R3 and R3xe2x80x2 independently represent an alkyl group having from 1 to 20 carbon atoms; and
R8O-(A4)x-CH2COOM2xe2x80x83xe2x80x83(4)
wherein R8 represents an alkyl group having from 1 to 20 carbon atoms; A4 represents an oxyethylene unit, an oxypropylene unit or a combination of an oxyethylene unit and an oxypropylene unit; M2 represents H, Li, Na, K or a group N+R9R10R11R12 in which R9, R10, R11 and R12 independently represent a hydrogen atom, a methyl group, an ethyl group, a 2-hydroxyethyl group or a 3-hydroxypropyl group; and x is an integer of from 1 to 12.
It is preferable that the pigment dispersion includes the compound having formula (1) and the compound having formula (2) (or formula (3) or (4)) at a ratio of from 99.99:0.01 to 60:40.
In addition, it is preferable that the pigment includes Pigment Yellow 138, Pigment Red 122, Pigment Blue 15, Pigment Blue 15:1, Pigment Blue 15:2, Pigment Blue 15:3, Pigment Blue 15:4 or a carbon black is used as the pigment.
The ratio (D/P) of the pigment (P) to the dispersant (D) is preferably from 15/1 to 1/1.
Further, it is preferable that the pigment particles dispersed in the pigment dispersion have a volume average particle diameter of from 20 nm to 200 nm.
The pigment is preferably included in the pigment dispersion in an amount of 5% to 30% by weight based on total weight of the pigment dispersion.
In another aspect of the present invention, an inkjet ink composition is provided which includes the pigment dispersion of the present invention mentioned above and an additive selected from the group consisting of water-soluble organic solvents, surfactants and antiseptics.
It is preferable that the pigment particles dispersed in the inkjet ink composition have a volume average particle diameter of from 20 nm to 200 nm.
In addition, it is preferable that the content of the pigment in the inkjet ink composition is from 1 to 8% by weight.
In yet another aspect of the present invention, an image forming method is provided which includes the steps of providing the inkjet ink composition of the present invention, and projecting the inkjet ink from a nozzle of a recording head to form an ink image on an receiving material. The image forming method preferably uses a piezoelectric inkjet recording head or a thermal inkjet recording head.
In a further aspect of the present invention, a method of manufacturing the inkjet ink composition is provided which includes steps of providing the pigment dispersion mentioned above; and adding at least one of water, water-soluble organic solvents, surfactants and antiseptics.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention.